The present invention relates to a method and apparatus for purifying impure liquid carbon dioxide (“CO2”) comprising a first contaminant selected from the group consisting of oxygen (“O2”) and carbon monoxide (“CO”). The process and apparatus have particular application in the recovery of carbon dioxide from waste carbon dioxide gas, for example flue gas from an oxyfuel combustion process or waste gas from a hydrogen (“H2”) pressure swing absorption (“PSA”) process.
There is an urgent need to develop new processes for production of electrical energy from fossil fuels, carbonaceous fuels or hydrocarbon fuels with capture of carbon dioxide. The new processes should ideally be more efficient and cost effective than existing processes. Oxyfuel combustion processes are being considered in this context.
In oxyfuel combustion, a fuel is combusted in pure oxygen with optional recycle of cooled flue gas or steam or water to moderate the flame temperature. The elimination of the bulk of the nitrogen from the combustion results in a net flue gas which has a high carbon dioxide concentration following cooling and water condensation.
An oxyfuel combustion process is ideally suited for use in a conventional pulverized coal fired boiler for generation of steam used for electric power production. The use of oxyfuel combustion in a pulverized coal fired boiler results in a net flue gas production which, after cooling and condensation of contained water vapor, typically comprises from about 65 mol % to about 95 mol % carbon dioxide and up to about 5 mol % oxygen with the majority of the remainder being nitrogen and argon. The oxygen, nitrogen and argon are referred to as “contaminant gases”.
The bulk of the oxygen in the flue gas originates from the excess oxygen required for complete coal combustion. The remaining oxygen originates from air leaking into the boiler and convection section. The nitrogen and argon in the flue gas originates from the oxygen feed for coal combustion, which would typically have a purity of 90 mol % to 99.6 mol %, and usually 95 mol % to 97 mol %, oxygen, and from air leaking into the boiler and convection section.
Also present in the flue gas are impurities such as acid gases and other impurities derived from the coal and the combustion process. The impurities include sulfur dioxide, sulfur trioxide, hydrogen fluoride, hydrogen chloride, nitric oxide, nitrogen dioxide, mercury, etc. The total amount of these impurities in the flue gas (after washing and drying) depends on the composition of the fuel and the combustion conditions.
The flue gas must be purified before carbon dioxide from the flue gas can be stored in, for example, geological formations. In this connection, water soluble components such as sulfur trioxide, hydrogen chloride and hydrogen fluoride, are usually removed from the flue gas by direct contact with water which not only washes out these components but also cools the flue gas and condenses water vapor. Sulfur dioxide and the oxides of nitrogen may be removed during compression of the carbon dioxide to pipeline pressure as disclosed in U.S. patent application Ser. No. 11/287,640 filed on 28th Nov. 2005, the disclosure of which is incorporated herein by reference. This process also removes any mercury that may be present in the carbon dioxide.
The pipeline pressure of carbon dioxide will usually be from about 100 bar to about 250 bar which is well above the critical pressure of carbon dioxide. The bulk of the contaminant gases is preferably removed to reduce the power required to compress the carbon dioxide and to ensure that two phase flow conditions do not arise in the pipeline or in the geological formation in which the carbon dioxide is to be stored.
The presence of oxygen may present problems when the carbon dioxide is intended for use in enhanced oil or gas recovery operations due to the possibility of oxidation causing corrosion problems in downhole equipment. The typical specifications for carbon dioxide purity would be a maximum contaminants level of 3 mol % and, in the case of the use of carbon dioxide for enhanced oil recovery, the maximum oxygen content would be typically 100 ppm or lower, even as low as 1 ppm.
The current technology for the next stage of carbon dioxide purification uses a technique in which the contaminant gases are removed from the compressed dried pre-purified crude carbon dioxide stream at about 30 bar pressure by cooling the crude carbon dioxide to a temperature very close to the freezing point of carbon dioxide, where the carbon dioxide partial pressure is from about 7 bar to about 8 bar. The residual gas, containing about 25 mol % carbon dioxide, is separated and vented after heating and work expansion to produce power. This single process results in a carbon dioxide recovery of about 90%. The process of oxyfuel combustion would be considerably improved if very high carbon dioxide recoveries, e.g. above 97%, could be achieved economically.
The current technology for delivery of carbon dioxide from the oxyfuel combustion of fossil fuel to a geological storage site is based on compression to a pipeline pressure of typically about 100 bar to about 250 bar. An alternative technology for smaller sources of carbon dioxide emission, or where a pipeline might be too expensive, is to liquefy the carbon dioxide and transport the carbon dioxide at a pressure below its critical pressure as a liquid in, for example, a large seaborne tanker. The oxyfuel combustion process would be significantly improved if the carbon dioxide purification process could produce economically a liquid carbon dioxide product rather than a stream of supercritical carbon dioxide at near ambient temperature for pipeline delivery.
An important objective for carbon capture in an oxyfuel power system is to provide a method of treating compressed crude carbon dioxide to remove nitrogen and argon and to reduce the concentration of oxygen to less than 100 ppm, preferably with low consumption of energy and high recovery of carbon dioxide. Carbon dioxide recovery (based on carbon dioxide in the total flue gas stream) should ideally be better than 97%. In addition, if the purified carbon dioxide product is produced as a low temperature liquid stream at a pressure below its critical pressure, transportation as a liquid or as a supercritical fluid to a carbon dioxide storage site is facilitated.
A further method of carbon dioxide capture from fossil fuels is to convert the fossil fuel to a mixture of carbon monoxide and hydrogen called synthesis gas (or “syngas”) by catalytic reforming with steam; by partial oxidation; by gas heated catalytic reforming; or by any combination of these known processes, followed by shift reaction of carbon monoxide and water to produce a net hydrogen-rich product gas containing carbon dioxide as the major impurity. These processes take place at high pressures, typically from about 20 bar to about 70 bar.
Hydrogen must be separated from impurities such as methane and carbon monoxide. Carbon monoxide must also be separated and purified. A preferred method of purification is to use a multi-bed pressure swing adsorption (“PSA”) process to produce a pure hydrogen. A typical PSA unit, operating at 25 bar pressure, would have a typical recovery of about 85% to about 90% of hydrogen in the feed gas. The composition of the waste gas, typically at a pressure of about 1.2 bar to about 1.5 bar, depends on the method used to produce the gas from the fossil fuel. For example, the PSA waste gas from a feed produced in a steam/natural gas catalytic reformer would typically comprise at least about 60 mol % carbon dioxide, together with lower quantities of hydrogen, methane, carbon monoxide and water vapor. In this case, the objective would be to reduce the levels of carbon monoxide and methane to below 100 ppm.
FIG. 1 depicts a flow sheet for a prior art process for removal of contaminant gases from crude carbon dioxide produced in an oxyfuel combustion process. The process is disclosed in “Carbon Dioxide Capture for Storage in Deep Geological Formations—Results from the CO2 Capture Project” (Capture and Separation of Carbon Dioxide from Combustion Sources; Vol. 1; Chapter 26; pp 451-475; Elsevier).
In FIG. 1, the carbon dioxide separation is carried out in a low temperature processing plant which uses carbon dioxide refrigeration to cool the crude carbon dioxide feed gas down to a temperature within about 2° C. of the carbon dioxide freezing temperature. At this point, a phase separation of the uncondensed gas takes place and the gas phase, containing about 25 mol % carbon dioxide and about 75 mol % contaminant gases is separated, warmed and work expanded to produce power before being vented to atmosphere.
The process separates the contaminant gases from the carbon dioxide at a temperature of −54.5° C. at a point close to the freezing temperature of the feed gas mixture, where the carbon dioxide vapor pressure is 7.4 bar. The refrigeration duty is provided by evaporating two streams of liquid carbon dioxide at pressure levels of 8.7 bar and 18.1 bar in heat exchangers E101 and E102. The two resultant carbon dioxide gas streams are fed to the carbon dioxide compressors, K101 and K102, which usually will be stages of a multistage compressor.
In FIG. 1, a feed 130 of carbonaceous fuel is combusted with a feed 132 of oxygen in an oxyfuel combustion unit R101 to produce a stream 134 of flue gas, the heat of which is used to generate steam in a power generation plant (not shown). Stream 134 is divided into a major part (stream 138) and a minor part (stream 136). Stream 138 is recycled to the oxyfuel combustion unit R101. Stream 136 of flue gas is washed with water in a gas-liquid contact vessel C105 to remove water soluble components and produce washed flue gas. A stream 142 of water is fed to the vessel C105 and a stream 144 of water comprising water soluble components from the flue gas is removed therefrom to provide a stream 146 of washed flue gas.
The stream 146 is compressed in a compressor K105 to produce a stream 1 of washed flue gas at a pressure of about 30 bar which is dried to a dewpoint of less than −60° C. in a pair of thermally regenerated desiccant driers C103 to produce a stream 2 of dried waste carbon dioxide gas. Stream 2 is cooled by indirect heat exchange in the heat exchanger E101 to about −23° C. to produce a stream 3 of crude gaseous carbon dioxide which is fed to a phase separation vessel C101 where it is separated to produce first carbon dioxide-enriched liquid and a first vapor containing the majority of the contaminant gases.
A stream 4 of first carbon dioxide-enriched liquid is reduced in pressure in valve V101 to about 18 bar to produce a stream 5 of reduced pressure first carbon dioxide-enriched liquid which is vaporized by indirect heat exchange in heat exchanger E101 to provide refrigeration and to produce a stream 6 of first carbon dioxide-enriched gas.
A stream 7 of first vapor from phase separator C101 is cooled by indirect heat exchange in the heat exchanger E102 to −54.5° C. to produce a stream 8 of partially condensed fluid which is fed to a second phase separation vessel C102 where it is separated into second carbon dioxide-enriched liquid and a second vapor, containing the majority of the remaining contaminant gases.
A stream 13 of second carbon dioxide-enriched liquid is warmed to a temperature of about −51° C. by indirect heat exchange in heat exchanger E102 to produce a stream 14 of warmed second carbon dioxide-enriched liquid which is reduced in pressure to 8.7 bar in valve V102 to produce a stream 15 of reduced pressure second carbon dioxide-enriched liquid. Stream 15 is vaporized and warmed by indirect heat exchange in the heat exchangers E101, E102 to provide refrigeration and produce a stream 16 of second carbon dioxide-enriched gas. The initial warming of stream 13 in heat exchanger E102 is critical to prevent freezing of the second carbon dioxide-enriched liquid on pressure reduction from about 30 bar.
A stream 9 of the second vapor from phase separator C102 is heated by indirect heat exchange to ambient temperature in the heat exchangers E101, E102 to produce a stream 10 of warmed second gas which is heated by indirect heat exchange in pre-heater E103 to about 300° C. to produce a stream 11 of pre-heated second gas. Stream 11 is work expanded in turbine K103 to produce power and a stream 12 of waste gas comprising about 25 mol % carbon dioxide and most of the contaminant gases which is then vented the atmosphere.
Stream 16 is compressed in the first stage K102 of a multi-stage centrifugal carbon dioxide compressor to produce a stream 17 of compressed carbon dioxide gas at a pressure of about 18 bar. Heat of compression is removed from stream 17 in an intercooler E104 using cooling water as the coolant. A stream 18 of cooled compressed carbon dioxide gas is combined with stream 6 and the combined stream is further compressed in the second or further stage(s) K101 of the compressor to produce a stream 19 of further compressed carbon dioxide gas at a pressure of about 110 bar. The concentration of carbon dioxide in stream 19 is about 96 mol %. Heat of compression is removed from stream 19 in an aftercooler E105 using boiler feed water and/or condensate as a coolant thereby heating the boiler feed water and/or condensate and producing a stream 20 of cooled further compressed carbon dioxide gas at pipeline pressure, e.g. at about 110 bar.
For simplicity, E101 and E102 are shown in FIG. 1 as separate heat exchangers. However, as would be appreciated by the skilled person, heat exchangers E101 and E102 would usually, in reality, form parts of the main heat exchanger with feed streams entering and product streams leaving at the most thermodynamically efficient locations. The main heat exchanger E101, E102 is usually a multi-stream plate-fin heat exchanger, preferably made from aluminum.
Table 1 is a heat and mass balance table for the process depicted in FIG. 1.
TABLE 1Stream Number12345678910Temperature° C.24.8324.83−22.66−22.66−30.8711.21−22.66−54.50−54.5011.21Pressurebar a303029.829.818.1263618.0263629.829.729.729.65Flowkg/s140.49140.40140.4027.7327.7327.73112.67112.6737.7537.75CompositionCO2mol %72.763372.865172.865197.605597.605597.605567.369567.369524.754624.7546N2mol %18.969418.995918.99591.50141.50141.501422.881922.881953.439253.4392Armol %2.69562.69942.69940.37120.37120.37123.21653.21656.90906.9090O2mol %5.43165.43925.43920.52180.52180.52186.53146.531414.896014.8960H2Omol %0.13960.00000.00000.00000.00000.00000.00000.00000.00000.0000SO2ppm0.00000.00000.00000.00000.00000.00000.00000.00000.00000.0000NOppm4.96744.97434.97430.69290.69290.69295.92545.925412.085912.0859NO2ppm0.00430.00430.00430.02100.02100.02100.00060.00060.00000.0000Stream Number11121314151617181920Temperature° C.300.0020.07−54.50−42.85−55.5011.2169.1725.00195.1043.00Pressurebar a29.651.129.729.658.7433218.54332118.1263618.02636110110Flowkg/s37.7537.7574.9274.9274.9274.9274.9274.92102.65102.65CompositionCO2mol %24.754624.754695.274795.274795.274795.274795.274795.274795.901295.9012N2mol %53.439253.43922.87232.87232.87232.87232.87232.87232.50382.5038Armol %6.90906.90900.79860.79860.79860.79860.79860.79860.68370.6837O2mol %14.896014.89601.05421.05421.05421.05421.05421.05420.91110.9111H2Omol %0.00000.00000.00000.00000.00000.00000.00000.00000.00000.0000SO2ppm0.00000.00000.00000.00000.00000.00000.00000.00000.00000.0000NOppm12.085912.08591.89131.89131.89131.89131.89131.89131.56921.5692NO2ppm0.00000.00000.00100.00100.00100.00100.00100.00100.00630.0063
The process depicted in FIG. 1 produces purified carbon dioxide having a carbon dioxide concentration of about 96 mol % and containing about 0.9 mol % oxygen at a carbon dioxide recovery of about 89%.
The general concept of using distillation to purify carbon dioxide produced in an oxyfuel combustion process is not new. In this connection, Allam et al (“A Study of the Extraction of CO2 from the Flue Gas of a 500 MW Pulverized Coal Fired Boiler”, Allam and Spilsbury; Energy Consers. Mgmt; Vol. 33; No. 5-8, pp 373-378; 1992) discloses a process for purifying carbon dioxide from an oxyfuel combustion process using distillation to purify the carbon dioxide to remove “heavy” impurities (such as sulfur dioxide and nitrogen dioxide), and contaminant gases including oxygen, nitrogen and argon.
In Allam et al, the carbon dioxide system is integrated with an air separation unit (“ASU”), using expansion of both the nitrogen and oxygen streams to provide refrigeration for the carbon dioxide liquefaction process. The process recycles part of the oxygen-containing stream separated from the carbon dioxide to the boiler, taking a purge stream at this point to prevent contaminants build up. A rectifying column is used at the cold end to remove lighter contaminants from the carbon dioxide stream. A second column, also at the cold end, removes sulfur dioxide and nitrogen oxides from the resultant carbon dioxide stream.
In addition, the general idea that a distillation column could be used to remove oxygen from carbon dioxide produced oxyfuel combustion process was disclosed by the Inventors in a paper entitled “Purification of Oxyfuel-Derived CO2 for Sequestration or EOR” presented at the 8th Greenhouse Gas Control Technologies conference (GHGT-8), Trondheim, in June 2006. However, no details regarding how the general idea might be implemented were disclosed.
Other prior art includes GB-A-2151597 (Duckett; published 1985) which describes a process of using membranes to concentrate a low concentration carbon dioxide feed stream so that it can be purified using phase separation. The aim is to make liquid carbon dioxide for sale rather than to recover as much carbon dioxide as possible from a combustion process and, accordingly, carbon dioxide recovery from the feed is very low at about 70%.
GB-A-2151597 discloses the use of the carbon dioxide feed stream to provide heat to the reboiler of the distillation column. GB-A-2151597 also discloses the use of an external refrigeration source to provide the liquid required for the distillation process to work.
U.S. Pat. No. 4,602,477 (Lucadamo; published July 1986) discloses a process for taking hydrocarbon offgas and increasing its value by separating it into a light hydrocarbon stream, a heavy hydrocarbon stream, and a waste carbon dioxide stream. The presence of the carbon dioxide in the stream decreases the heating and economic value of the gas. The process uses a carbon dioxide membrane unit to perform a final removal of carbon dioxide from the light hydrocarbon product, in addition to a distillation step performed at low temperatures.
The aim of the process disclosed in U.S. Pat. No. 4,602,477 is not to produce high purity carbon dioxide but to remove carbon dioxide from the hydrocarbon feed. The distillation step produces the carbon dioxide stream as a side stream from a rectifying column having a condenser. The process also uses a stripping column to purify the heavy hydrocarbon stream.
U.S. Pat. No. 4,977,745 (Heichberger; published in December 1990) discloses a process for purifying a feed stream having a carbon dioxide feed purity of greater than 85 mol %. The high pressure residual stream is heated and expanded to recover power but an external refrigeration source is used to liquefy the carbon dioxide.
EP-A-0964215 (Novakand et al; published in December 1999) discloses the recovery of carbon dioxide from a process using carbon dioxide to freeze food. The process involves the use of a distillation column to recover the carbon dioxide. The carbon dioxide feed stream to the column provides reboiler duty to the column before being fed to the column as reflux.
U.S. Pat. No. 4,952,223 (Kirshnamurthy et al; published in August 1990) discloses a carbon dioxide liquefaction process in which the carbon dioxide recovery is improved by passing the vent gas to a PSA system to produce a carbon dioxide-enriched recycle stream and a carbon dioxide-depleted vent stream.